What defines a thermodynamic system?

A thermodynamic system is fundamentally defined as a part of the universe that can exchange energy and/or matter with its surroundings. This definition encompasses various types of systems, such as open, closed, and isolated systems, which can interact differently with their environment.

In a thermodynamic context, an open system can exchange both energy and matter, while a closed system can only exchange energy. An isolated system, though not able to exchange either, is still considered within the framework of thermodynamics. The ability to exchange energy and matter is vital to the study of thermodynamics because it allows us to analyze processes such as heat transfer, chemical reactions, and phase changes.

In contrast, the other choices do not fully capture the nature of a thermodynamic system. A completely isolated portion of matter does not allow for any interaction with its surroundings and thus is a specific case rather than a general definition. A substance at its melting point describes a condition rather than the system itself. Lastly, an arrangement of gases with no interactions fails to acknowledge the significance of energy exchange, which is a core principle in thermodynamics.